Now yes, you can use that 2.5-inch drive bay to house that old hard drive from your PlayStation 3 (before you maxed it out). However, the reason that it is there, is for SSD storage.
iMac is designed to not need any external storage. You can have 2 TB of storage, and back it up to another iMac somewhere else in your house. That’s not to say that you can’t have an external hard drive, it does have both USB 2.0 and FireWire… I’m just outlining Apple’s vision. With a 2.5-inch SSD drive, you can boot from SSD but store files internally on the larger hard drive.
And, I suspect, that is the future of MacBook Pro. With storage manufacturers flocking to the new MacBook Air blade-type storage format, I suspect MacBook Pro will tout it as an optional upgrade as well. Unlike a 2.5-inch drive, it can fit inside the MacBook Pro chassis with minimal modification.
And, with Mac OS X Lion getting TRIM support, SSDs on Mac will finally work properly. So, really, I’ll be more surprised if MacBook Pro doesn’t adopt SSD. This is one key feature that will finally allow Apple Pro Portables to stand out once again, and re-justify their hefty price tags. It will also help turn the tide away from the 13-inch MacBook, and back to the 13-inch MacBook Pro. While the Pro variant has been outselling the plastic non-Pro variant, it mostly has done so because of its shiny metal posterior… and not on technical prowess.
You can achieve this today, with a few different products that trade the optical drive for a second 2.5-inch slot. However, not only do you lose your optical bay, but you also have to deal with installation costs (including time) and maintenance (such as, pulling the drive when you need to send your MacBook in for care).